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Will the steep beta descent mode become illegal for new aircraft?
The PC6 is a popular skydiver carrier due to its ability to use steep nose down beta descent to get back to the ground quickly, even beating free falling skydivers.

(YouTube)
FAA AC25.1155x (I only saw the draft) proposes that a "means to prevent intentional or inadvertent selected of reverse thrust or propeller pitch below the flight regime" be required, which cannot be overridden. The intention is to prevent several accidents resulting from said deployment. The lockout means must not degrade landing performance, so the intent is not to eliminate beta entirely.
Does this mean that no new aircraft can be certified that has a descent mode from altitude similar to the PC6?

What I saw was Part 25. It was old as well.

I wouldn't call it a "descent mode from altitude".  It's considered to be a dangerous and foolish practice to operate an engine in ground beta (generally coming back into DISCING - blades flat) while in flight to get steep descent rates.  If the prop doesn't want to come out of ground beta, you are dead meat.
When the Twin Otter was in production, it was an open secret that demo pilots were using DISCING to get crazy steep approaches to impress customers.  After a couple exciting events, flight ops finally put an end to it.
Some airplanes already have ground beta lockout systems.  Is this a proposal to mandate a system for the PC6 specifically?

What I read was not PC6 specific.

If the prop doesn't want to come out of ground beta, you stop the fuel flow and glide to a landing. A better precaution against pilot incompetence is better training, not more system complexity.

If you shut the engine down and the prop moves to feather on its own, the normal action, that's fine.  If a prop gets stuck at or near DISCING and you shut it down, and the prop stays put, you will still be coming down like a crowbar with a flat pitch prop windmilling and the result will be unpleasant.  That said, I agree that it's an airmanship issue (as in, a practice that is really stupid, airmanship wise) and not something that requires Beta lockouts on small turboprops.

I found this FAA Working Group paper on it
https://www.faa.gov/regulations_policies/rulemaking/committees/documents/media/taepiht14-112699.pdf
(28 page paper, poorly scanned PDF, I am not copying any sections here.)
It looks to me under the Recommendation section that are proposing putting the plane into beta just be made a seperate distinct action such that the crew cannot accidentally go into beta. They must go to Flight Idle, then beta could be allowed.
I am not seeing any follow up either, so I don't know if the AC was published.

Most aircraft have a mechanical detent that provides an idle stop, and has to be disengaged, like a trigger or a gate.  The Twin Otter has a gate at flight idle, basically a step in the track, and you rotate the hand grips to bypass it to come back into ground beta.   I can't imagine an airplane with at least that kind of action required.

@JohnK Can the gate be bypassed in the air or just on the ground?

In the air as well unless the aircraft has a Ground Beta lockout system which some larger complex turboprops have.  Most I would say don't have it.  A Twin Otter certainly doesn't and you can go into full reverse in the air if the mood suits you.

I just came across this. AOPA is a reliable source but it is not a reference to a regulation.

Beta mode is only available for ground operations. Many single-engine turboprops have low propeller ground clearances, so it is vital to minimize beta thrust in contaminated areas to avoid engine and prop damage from dirt and debris. A few creative pilots have tried using beta thrust in flight to increase descent rates; however, some of those who have tried that trick wound up at the bottom of a smoking hole. The use of reverse thrust in flight is strictly prohibited in virtually every type of aircraft. That’s why most turboprop propeller controls have in-flight reverse-thrust lockout systems. Unless it’s approved for your aircraft, don’t even think about it.

Oh, come on. You can still stop the engine and glide to a safe landing. No smoking hole required in case of beta malfunction.

@PeterKämpf  I agree with that. After reading the quote a couple more times I still don't see that it's possible to get in flight beta approved for new aircraft, only that some aircraft have been approved.

Splitting hairs practically speaking, but there IS a flight Beta mode on turbo props now.  The transfer of blade angle control from the prop governor to the power lever, and rpm from the prop governor to a fuel controller based rpm governor, which is what Beta mode is (similar to a turbine helicopter really), happens just ahead of IDLE.  So when you are at or just above IDLE you are in "flight beta".


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Are there any "captains-only" airports in the U.S.?
Some airports in the world only allow captains to take off or land the aircraft, due to airport design or environmental conditions (obstacles, high winds).
Examples include:

Funchal (Madeira, Portugal)
Heraklion (Greece)
Paro Paro (Bhutan)

Are there any airports in the United States that are "captain-only", either for take-off or landing or both? Why so?

It's not the airports that make them Captain-only, it's the specific operators. ie Airline-A may have that airport as Captain-only, but Airline-B may not. Their choice.

@RAC Not always true. I know that India has several airports that are designated as captains-only by the regulatory authority

I suppose that instrument approach and departure procedures that require specific authorization from the FAA (obtained by the Operator) come the closest.

To RAC's point, I know that some Airlines require pilot's to be signed off for Jackson Hole. I don't recall if it is a captain's only rule or not. But they have to be specifically trained on the airport a fly multiple flights with an internal check airman.

Some airlines probably want pilots who can be trusted with large bundles of cash at Jackson Hole.  The fuel prices there are huge.

As already mentioned in the comments, I don't think there is such a thing as "captains-only" airports. The airline/operator might impose special requirements on crew, such as a certain amount of experience and/or being a captain. It might also require the crew to be additionally trained for that specific airport (this can for instance be conducted during the Operators Proficiency Check).
To add to that, I could be captian on one flight, and be first-officer on the next. Makes no sense to not allow me to conduct the approach/departure, just because my rank is not captain on that specific flight.
In Europe, I know of some airports in the Alps (and Cannes on the French coast, just to name one) that require a yearly theoretical test to be made by pilots operating on the airport. This is due to some very specific rules at those airports.

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How did a Soviet SA-3 missile manage to shoot down the F-117 Nighthawk?
This question is about the F-117A that was shot down in 1999. How did an obsolete missile like the SA-3 shoot down such an advanced aircraft for its time? This question has nothing to do with how aircraft have become more stealthy. It is specific to the 1999 incident. I am just curious as to how an aircraft this stealthy could have been shot down.

Explained in part of this answer

"Stealth" does not mean invisible. In fact, there are photographs of F-117s, so clearly, they are visible.
Stealth means less observable than a comparable non-stealth design. A stealth aircraft still has a radar signature, it still generates heat, it still generates noise, and it is still visible … just less so than a non-stealth aircraft.
In this particular case, there was a combination of several factors:

The F-117 was the first production stealth aircraft. A lot of things weren't known by then, for example how to integrate sensor pods, radomes, etc. into the faceted hull in such a way as to not negate the stealth characteristics. As a result, the F-117 was pretty starved for sensor data, for example, it didn't have a radar warning system. The pilot literally didn't even know that he was overflying a missile station until he saw the missiles coming at him through the clouds.
The F-117s were flying exactly the same route at more or less exactly the same time night after night. That's just shoddy mission planning.
The bomb bay is a huge radar reflector, with the doors open, the plane is no longer stealthy. (This is still true for the B-2, BTW.)

So, since they flew the same route every night, it was likely that someone, somewhere would just simply spot or hear them by accident. No matter how stealthy your aircraft is, if someone looks up and you pass between them and, say, the moon, they will see you. Since the route was known, the Yugoslav Army could move their missile station right into the flight path. Since they were also repeatedly bombing the same targets, the Yugoslavs could even make an educated guess at which point the bomb bay doors would be open. They also only had to turn on their radars for short periods of time (again, since they already knew roughly when and where the airplane would be), making it harder to detect them.
And once the missile hits you, it doesn't really matter how old it is. Kaboom is kaboom, regardless of age.
So, to summarize, it was a combination of

bad mission planning
limited sensor capabilities of the F-117
a very clever missile commander (credit where credit is due!)

It was even worse. The F-117 FCS allows it to fly with utmost precision. Doing this several days in sequence simply meant that the Serbs only had to control their watches and time the missile launch accordingly.

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Is wing flex good?
I've noticed that the wings of the 787 and A380 tend to flex a lot. Does wing flex help an aircraft in any way?

Not flexing would be a bad thing.  The flexing absorbs a lot of the loads on the wing.  Look at the picture above.  Now imagine that the wings were rigid.  Imagine the internal structure required to keep them rigid and how the loads would be spread through the structure.

See also this question

I'm surprised none of the answers so far discuss any cyclical fatigue. When I see flexing wings, the first thing i think of is how much more bending can they take before they snap like a paperclip?

As long as it flexes, it doesn't break.

In this world any physical material that is stressed will strain( flex). The lower the modulus the more the strain. Presumably the engineers have determined how much is acceptable without impairing the function.

The wings of the Boeing 787 are so flexible because its carbon fiber material can be stretched more, and the high aspect ratio of 11 will magnify this effect. In flight the consequences are:

Less shaking due to gusts, because the wing will dampen load changes more effectively.
Delayed aileron response, because the lift change due to aileron deflection will first bend the wing before it starts to roll the aircraft.

On the ground one consequence is:

The wing might have less tip clearance, because less in-built dihedral is needed - the rest is supplied by the wing's elasticity in flight.

In general, wing flex is like the suspension in a car. It costs a little bit of performance but gives a much smoother ride.

@Dave: We are not comparing blocks of the same size, but structures made to carry the same load. And if you look at http://www.christinedemerchant.com/carboncharacteristics.html, carbon fibre composite is ~20 times stronger than aluminium. While it only has ~2.5 times higher elastic modulus. The strength numbers are by weight while the elastic modulus is by cross-section, so it has to be corrected for difference in density and the best aluminium alloys used are probably better than the figures listed, but still indicates that the difference in strength is bigger than difference in rigidity.

@Dave: So a structure built to carry the same load can be lighter by more than the difference in elastic moduli and as a result will flex more. That is what “can be stretched more” means.

Carbon fiber does bend a lot more for the same strength. A rule for thumb for carbon fiber is that when it is rigid enough that you feel safe (like a bicycle for example) it is safe.

My apologies I have always been under the impression that it did not bend that much (at least from my experience with it) I will remove my comment to reflect my error.

I will try to add over @PeterKampf answer. Flexibility is finally another parameter, when you make your design allowing your airplane to be flexible, you are introducing a new variable. As in all optimization problems adding new (smart) parameters allows you to create a more optimized design.
Just taking the example provided by @PeterKampf , if the airplane is less shaking due to gusts the requirement over the aerodynamics/structure of gust response is more easily achieved. That makes relaxation in some areas of the structure making it lighter. So, altohugh there is a potential cost in aerodynamic performance, it might end in savings in fuel as the airplane will be lighter for the same gust response.
There is also an effect that is not seen in the picture, what you is the flexion produced by the lift, but also the lift is producing cambering, which can be also exploited by the wing to have a more optimized design.

In case of the 787, its flexibility was not a free design parameter, but the consequence of the material, the low airfoil thickness for flight at Mach 0.85 and the high aspect ratio. And to my knowledge cambering only manifests itself in hang glider wings - here the cover is flexible enough to be pulled out by aerodynamic suction.

Flexibility is a parameter.... and the decision is to use that material and  reduce thickness of the airfoil. Is the other way arpund.

Do you honestly think that cabonfiber was selected because it produces more flexible wings? Or was it selected because its lower mass, and the higher flexibility was accepted?

We are saying the same thing here... traditional design was avoiding flexibility, in order to accept flexibility a benefit in weight was obtained in exchange. My point is that we add another parameter or removing one constrain. Making the airplane more flexible. We are discussing if allowing flexibility from aerodynamic point of view was the enabler or a new material put pressure on aerodynamic for accepting the material. Is chicken-egg question. It is both

@PeterKämpf they could have changed the parameters of the wing panels and struts to make them more rigid, but didn't to prevent overstressing the material.

Can you explain in a bit more detail what you mean by "the lift is producing cambering"?  It's not obvious to me that this is so.

To add to the other answers, a stiff wing is a heavy wing. If flex can be permitted then the wing can be made lighter. Flex does increase the risks of things like fatigue cracking (metal), delamination (composite), excessive strain and aerodynamic surprises, but if these are all allowed for in the design then the plane will be lighter and have a better payload-range than the equivalent stiff wing.

Wing flex is also to increase the aerodynamics of the plane. Wing flexes produce more lift because of their flexibility which actually allows more lift to generate. Wing flex acts as flaps but isn't, wing flexes can also support thousands of pounds and lift them with ease because of the lift it generates. This is why the Boeing 787 doesn't need flaps extended to the required curve for takeoff due to the curvature and flex of the wing.

welcome to aviation.se. may I ask you if you have any source for what you wrote? thank you!

I have no source, it is just pure knowledge on plane aerodynamics and how it helps an aircraft produce lift

Would you then back up this knowledge with an explanation on how exactly a spanwise curvature performs the same task as a flaps extension?

Unfortunately I can’t find any evidence that your statement is true and is counter intuitive to everything I know about aerodynamics.


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Why do certain airlines give names to their aircraft?
That is the case for aircraft operated by e.g. KLM. Some aircraft are named after birds, some after cities. Is that common for other airlines as well or something that only KLM does?

The German Lufthansa also uses city names for their aircrafts, and I also saw Arabic names on aircrafts from Tunis Air.

I know all TG crafts are named. It is a form of honoring the name of city or places. I believe Qantas also do this.

If I paid $300 million for something, I would definitely be giving it a name.

JetBlue names their aircraft also

And Virgin, Jet2 and Frontier.

Pretty sure this predates aircraft. Most every ship has a name, not just a registration number.  And, for marketing purposes, a lot of railroads have "names" for trains plying certain routes.  "Wabash Cannonball," anyone?

The practice of naming aircraft probably comes from the days of the flying boat, and the naval tradition of naming ships - the Pan Am Clippers had many named aircraft, and Pan Am kept up the tradition into the Jet Age. Other airlines of the period had similar naming practices (I'm not sure who started it, but Pan Am's fleet was well known for it).
As a marketing tool telling folks they're flying on "Clipper Emerald Isle from New York to Dublin" may have been more attractive to passengers than "Pan Am Flight 12345" or N880PA. Particularly before the commoditization of air travel, when flying was still a new and novel form of transportation, having a connection back to familiar "sailing" metaphors may have helped get passengers on board and could help differentiate your service from another airline.
Modern airlines don't seem to push their aircraft naming as much as historic carriers did, but as SentryRaven noted there are a few airlines that still name their ships - in fact Lufthansa paints their plane names onto the fuselage, though I'm not sure if they announce them to passengers.
A skilled marketing department could build a brand around ship names, but I'm not aware of any airline doing so today.

Spicejet in India does just that - build a brand around their jet names. Their planes aren't named very well - one of them being named "Turmeric" (a spice), but all their airplanes share spice name.

The KLM DC-2 "Uiver" (1934) predates the B-314 Clippers (1938-1941) by at least 4 years. But Pan Am also flew S-40 under "Clipper" nicknames, back in 1931.

Honestly, I didn't knew that there are airlines which doesn't give names to their planes. Maybe because I'm from germany. Even my car has a 'name'.

There are also other airlines which name their aircraft, e.g. Lufthansa.
To understand where this comes from, I recommend reading the blogs below, excerpts quoted here for a quick intro.

Lufthansa | Aircraft Naming

Lufthansa naming conventions as a sign of our time
The "Herborn" brings the total number of aircraft named after German cities and states to over 300. This naming convention has a long tradition at Lufthansa. The first Lufthansa aircraft was named in 1960. The idea was to express the company's solidarity with its German homeland – not just with the major hubs and cities but also with the regions where a large portion of Lufthansa passengers and employees come from.

Meanwhile at KLM | Every Plane Needs A Name

KLM has always had a rather unique approach to naming its planes. It’s unclear who first came up with the idea of giving names to aircraft, but we do know that our first aircraft got its name in a completely different way than those in our current fleet.
  KLM’s first Koolhoven actually had a nickname, “Dikke Dirk” (Fat Dirk), which was inspired by the last two letters of its registration, H-NADD, but also aptly described the unusually chubby fuselage of this aircraft type. “Dikke Dirk” was followed by “Piet Haas” (Peter Hare), because by then the Netherlands adopted the PH registration, which is how the Koolhoven registered PH-AES got the name “Piet Haas” (“haes” closely resembles “haas”, the Dutch word for “hare”). All this is much the same, I suppose, as people naming their car after the letters in its registration.
  Let’s move on to the first “real names”. Not surprisingly, bird names were very popular. Vast flocks of bird names, in fact. Most KLM staff and aficionados are familiar with the “Uiver” (Stork), “Snip” (Snipe) and “Pelikaan” (Pelican), which all made legendary flights. The name “Uiver” certainly harks back to the era in which the plane was named, because most Dutch people nowadays would call a stork an “ooievaar”.

Meanwhile at KLM | Who Thinks Up Aircraft Names

What do the “Borobudur”, “Museum Square” and “Blue Kestrel” have in common? They all crisscross the globe proudly displayed on KLM aircraft. But why do KLM aircraft have names? And who thinks them up?
KLM is one of the few airlines that gives its aircraft names. And I have to admit that I was unable to find out why. What I do know is that our very first aircraft, a Koolhoven FK-33 acquired in 1925, was known as the “Dikke Dirk” (Fat Dirk). The aircraft did have an unusually broad waistline, but the name had more to do with the last letters of its registration: DD.
  What are KLM’s aircraft named after?

  Airbus A330s: City Squares Worldwide
  Boeing 737s: Birds
  Boeing 777s: World Heritage Sites
  Boeing 747s: World Cities
  Boeing 747 Freighters: Ships of the Dutch East India Company (VOC)
  Boeing 787-9 Dreamliners: Flowers

Southwest Airlines name some but not all of their aircraft; these are typically either limited runs for marketing (such as the Sea World, NBA and Sports Illustrated partnerships), aircraft dedicated to a particular state in Southwest's "home territory" (Arizona One, California One, Missouri One, New Mexico One, Lone Star One, etc), or else the first planes in some significant change to Southwest's fleet (Heart One and Heart Two were the first to be painted in the Heart livery that replaced the earlier Canyon Blue; the first Canyon Blue livery, in turn, was borne by Spirit One). A few are "signature planes" dedicated to notable Southwest employees or aviation figures in general, such as Fred J Jones. The full list of special liveries is on Wikipedia.

names are one thing, liveries are another.

In SW's case they're more or less one and the same; Heart One and Heart Two are expected to have those names, and the liveries, until the airframes are retired, even if Southwest adopts another livery. The same applies to Spirit One and the state liveries. Only the marketing-based ones didn't get special names.


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Can astronavigation be used during flight?
In an aircraft, can you look at the sky to get your bearing and position like sailors do?

The Zeppelins used a sextant quite often. In the times before radionavigation that was the best they could do when ground was obscured or of no help (like over deserts or open water).

In Canada knowledge of how to get a fix with a sextant was included in the Airline Transport exam until the 80s.

Yes, but for most of us, it's far more useful to look at the ground. E.g. IFR = I Follow Roads :-)

Related: https://aviation.stackexchange.com/questions/25096/what-is-the-purpose-of-eyebrow-windows

You sure can! It's not really done all too much any more in the days of GPS but it was done quite a bit in the early days of aviation. Historically, this was done by the "flight navigator" a position that no longer really exists. Some aircraft even had mounted sextants.

(source)
Here is a video of it being done in practice (fanta optional)
This was even automated on the SR-71's Inertial Astro-Navigation system which used the stars to align itself.

The SR-71 operated in the 1960s and 1970s, long before high-resolution light-sensors (i.e. CCD/CMOS sensors) and computer-systems powerful enough to process a raster graphics of the night-sky existed (let alone a frame-buffer big enough) - so on what basis did the SR-71's NAS-14V2 operate? Surely there's too many bright stars visible in the sky for a computer system back then to reliably know which star it's looking at from what I assume was a 1960s-era vidicon tube output signal?

@Dai You don't need a computer to process raster graphics. The Russians sent back images of the moon in 1959. They scanned an analog TV camera and converted it to a fax signal. The format of the images was basically fax. You can do something similar to process star locations - point an analog TV camera at the sky and process the signal in analog. Of course, a standard analog TV signal is essentially 320x240 but a custom camera can be any resolution you're willing to process

@Dai you're assuming that a general-purpose digital computer is required. Dedicated analog computers can sometimes achieve performance levels that general-purpose computers can't.

@Dai if you are curious the full POH has been declassified, containing operational instructions. There is not all that much out there on how it worked but if you watch some of the -71 interviews they talk about it tacking certain bright stars.

@Dai here is another copy of the manual that discusses a bit more in depth how the unit works

@Dai: While I know nothing about the SR-71's system, it was quite common for space probes to orient themselves by tracking bright stars.  Apparently it was also used for missiles, so there would seem to be no reason such a system couldn't be used on a manned aircraft: https://en.wikipedia.org/wiki/Star_tracker

Basically the trick to doing astronavigation with limited computing power is not to attempt to process a raster image. Instead there is a small telescope with a narrow field of view, focusing onto a quadrant photodetector which you could think of as being similar to a CCD with just 4 pixels. This telescope is initially aimed either manually or automatically (based on an existing INS alignment) at a star selected from a small catalog. The relative intensities of the 4 pixels provide a correction signal that indicates how far and in what direction the star is from the center of the viewfield. ..

... and the computer then steers the telescope so as to null out this correction signal (all 4 quadrants at equal brightness). This condition is maintained continuously, "tracking the star" as the aircraft attitude and position changes. The gimbal angles necessary to do this provide a measurement of the line-of-sight vector to the star, which is used in the navigation solution.

@pericynthion In a dogfighting situation - or anything with short, sharp maneuvers - I don't see the gimbal being able to react fast enough to track the star like that. Was astronavigation only available when flying in a straight-line?

@Dai I doubt any automated astronavigation systems would have been installed on aircraft that engaged in dogfights.

It has in the past. Though the process for both pilots and sailors is a little more complex than just looking at the stars. See a previous AviationStackExchange question.
Do pilots need to know how to use a sextant?
Modern aviation has so many other navigation tools:
GPS
Radio NavAids
Inertial Navigation Systems
They make Celestial navigation obsolete. Although, it is not required pilot training. And you won’t see it used in GA aircraft. It is still in use by the military. Especially in ICBMs. My understanding is that the automated celestial navigation system used by the military was too expensive for widespread use.
Chapters 8-13 of the FAA Flight Navigator Handbook are dedicated to the subject. Sadly, the position of a Flight Navigator is also all but obsolete.

FWIW, I spoke with a chief engineer about how often all navigation sources you listed simultaneously fail, and the answer was that you might never see it happen over an entire fleet's lifetime.  The only cases I've seen in ASRS of total position loss are for AHRS planes.

Of course it can be done, and astronavigation was often used in long-range flights, and transatlantic passenger liners such as the Super Constellation were fitted with a plexiglas astro-dome so that the navigator might 'take' the stars with a bubble sextant.


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The SR-71 flew at 2193 mph. Mathematically, how can this be Mach 3.3?
Virtually every time I read about the Lockheed SR-71 Blackbird (including other threads on this forum), I see that its speed record is listed as approximately $2193$ mph.  The same blurb will invariably say this is "about Mach $3.3$". However, when I search for how fast Mach $1$ is, the number that almost always comes up is $767$ mph. Now, $767 \cdot 3.3 \approx 2531$, which is well above $2193$.  If I take $2193$ and divide it by $767$, I get Mach $2.85$.
I doubt I've stumbled on something new here.  So what am I missing? How can $2193$ mph be Mach $3.3$ when $767$ mph is Mach $1$?

related What is the reason for changing the speed reference (IAS or Mach number) with altitude?

great question, thanks @Joe - whilst I knew that mach is meant to vary depending on air conditions, I did not know the details - and now we have all the details!

Well you see, multiplication works differently at high altitude..

Because Mach number as a function of temperature, not physical speed

The SR-71 was only capable of Mach 3.3 flight at altitude. You can see the limitations in the pilot's operating handbook(POH) for the aircraft:

At higher altitudes the relative speed of sound is lower. If we use this handy NASA calculator in a standard atmosphere the relative speed of sound at 70,000 ft. is 660 MPH making your 2193 MPH equal to Mach 3.321

Its also important to note that the word "about" is often used as the SR-71 was flown to a maximum compressor inlet temperature(CIT) not a maximum Mach when doing a 3.3 run. According to the POH

Mach 3.2 is the design Mach number. Mach 3.17 is the maximum scheduled
  cruise speed recommended for normal operations. However, when
  authorized by the Commander, Speeds up to Mach 3.3 may be flown if the
  limit CIT of 427°C is not exceeded.

Comments are not for extended discussion; this conversation has been moved to chat.


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Where does the 35 feet screen height come from?
When taking off, why are we required to climb at least 35 feet over the departure end of runway? Why not other numbers? This comes from the FAA regulations. I believe there must a clear explanation for this but can't find one. Is there any historical or mathematical reason?

Where do you get that? A lot of books I've read have the mythical "50 foot obstacle". Perhaps you are referring to clearing the approach lights?

@RonBeyer I guess he's asking about the minimum performance conditions for obstacle clearance in the US

35' is probably just a random number that someone thought was reasonably attainable for most aircraft. For a wet runway the figure is reduced to 15', and for propeller powered aircraft it is raised to 50'.

My guess is that since most (all?) aircraft provide performance data to clear a 50 foot obstacle at DER, the FAA selected 35 feet in order to allow pilots to more easily calculate required aircraft performance while still providing an adequate safety margin between the two.

Part 23 requires 50 feet (Normal, utility, acrobatic, and commuter categories) while Part 25 requires only 35 feet (transport category)

Yeah I'm asking about FAA regulations. So why those numbers are set ? Those cannot be just random numbers, there must be logical reasons.

Well, unfortunately that is what I'm looking for :) It could be a random number set by FAA. However I'm not going to be happy for being a pilot if I find out that FAA guys set numbers by rolling a dice. Especially when it comes to safety issues such as take off. Thanks for wish me a luck sir!

The best explanation I've seen for the logic behind the 35 foot crossing height requirement can be found in the FAA Instrument Procedures Handbook (IPH). It's available from the FAA website at this link: https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/instrument_procedures_handbook/
In Chapter 1, Departure Procedures, under Design Criteria on page 1-16: "The aircraft climb path assumption provides a minimum of 35 feet of additional obstacle clearance above the required obstacle clearance (ROC), from the DER outward, to absorb variations ranging from the distance of the static source to the landing gear, to differences in establishing the minimum 200 FPNM climb gradient, etc."
So, the 35 feet is in place to account for a multitude of variables. The phrase "differences in establishing the minimum 200 FPNM (Feet Per Nautical Mile) climb gradient" covers a whole host of potential errors such as variations in individual aircraft performance, early rotation, late rotation, under rotation, over rotation, poor airspeed control, etc. This error proofing is important since the 35 foot requirement is used when building all performance data for go and no-go decisions. That performance data is all based on set assumptions. Reasonable deviations from the assumptions are mitigated (somewhat) by the 35 foot margin.
As with most safety margins, there isn't necessarily a precise reason for the size of the buffer. However, even increasing the crossing height a slight amount, say to 50 feet can have significant effect on performance calculations. At a minimum climb gradient of 200 FPNM, the difference between a screen height of 35 feet and 50 feet is just over 450 feet of required horizontal distance. In a tight performance situation such as high pressure elevation or short runways, a 50 foot crossing height could be too much for some large commercial aircraft.
Of note, the regulation that created the 35 foot crossing height, SR422, came about in the late 50s during the beginning of jet-powered commercial aviation. The aircraft being designed and fielded back then, the Boeing 707 for example, would have had a hard time making a higher restriction. That likely had influence on the required crossing height.

"35' is the best our latest and greatest whiz-bang toy can do, so that's the requirement we're going to set." Sounds quite governmental!

and 4 years later I noticed "FPMN" and "FPNM". "Feet Per MiNute" or "Feet Per Nautical Mile", or something entirely different?

@FreeMan It's a climb gradient so it's rise over run, or Feet Per Nautical Mile. I added the acronym description for clarification.

"FPMN (Feet Per Nautical Mile)" :D  I fixed it for ya... You had "FPNM" correct later on.

Screen height is based on the height of a (London) "double decker bus, as used in the original trials at Croydon, where they hit one while taking off."
Source: Phil Croucher (2019) EASA Professional Pilot Studies, p.9-6

Well, why didn't they write a regulation that you weren't allowed to park a double decker bus at the DER? That would have made more sense! Nice find.

According to Wikipedia the height of a double-decker bus in the UK is 16 ft 3 in, so 35 feet would be more like the height of a quadruple-decker bus.  (I'd like to see one of those.)

This doesn't really make sense. That would be a three story tall bus. Not to mention the question of why the FAA would be conducting some sort of initial trials regarding instrument procedure design in London.

As I understand it, from when I was attending flight engineer school with the Air Force, the screen height has to do with a 50' obstacle. The 35' screen height for FAA TERPSd fields translates to being able to clear a 50' obstacle at 200ft/min providing you are 35' AGL at brick last of the runway.
For Army TERPSd fields the screen height is typically 16' and for Air Force and Navy TERPSd fields the screen height is 0', meaning that if you maintain 200ft/min climb, you can rotate at brick last and then you will clear the 50' obstacle by the required minimum clearance.  The 35', 16' and 0' screen heights are assumed if no other screen height is given, and based upon who has evaluated the airport.


text:
What extreme measures might a pilot take to avoid extreme weather?
What kind of extreme weather would justify a pilot of a commercial airliner taking avoidance measures to the point of likely stall?

You are assuming that the events leading to the "point of likely stall" are deliberate and controlled.  In real world cases where this scenario has been a contributory factor, this has not been the case.  No sane pilot is going to deliberately fly into a stall unless training.

Any which the pilot thinks is more dangerous than the avoidance measures?
If you were driving at 100mph towards a wall which you reckoned had a 90% chance of a head-on collision, or could attempt to turn which had a 50% chance of throwing you off a cliff, which would you choose?
Any decision the pilot makes will be made on the basis of "Taking this action is a better option than not taking it".
The main weather which would likely cause this would be high enough levels of icing to be dangerous, or such severe turbulence that the pilot believes the plane will be "upset" enough to throw it out of the performance envelope. A combination of these may be present in a severe storm.

Thanks for your answer Jon. I assumed that there must be weather conditions which were so dangerous as to warrant a relatively risky course of action to avoid them.  My interest was in what those conditions might be. A point which you have addressed in your last paragraph. Thanks again.

Possibly debatable, but not just turbulence.  Severe up and down draughts can overwhelm an aircraft's ability to fly out of them.  I see these as distinct from turbulence.

I see what you mean, in that Turbulence is chaotic whereas an updraught/downdraught system is on a larger scale so appears more "ordered", but both tend to come as part of a storm and are really just a type of turbulence (ie chaotic air movement). Also, attempting to climb out of or over an updraught is somewhat defeating the object.... either way, that seems to me to be arguing semantics, not principles.

@JonStory a very strong mountain can be very difficult to fly out of, especially in the Q corner.

Yes I definitely suggest not attempting to fly out of (or into, or through) any mountains

Sorry word left off. "Mountain wave"

That does make a fairly significant difference :) although I maintain that the original answer stands: the pilot will climb where s/he feels that the risk of climbing over a weather condition is lower than the risk of passing through/staying in/going around it it.

If you are talking about the AirAsia Crash, you may have some misapprehensions.
Normally, the problem with a storm is not "avoidance" once you are in it. The problem is keeping the aircraft flying in a stable configuration in the presence of violent winds. A powerful enough wind can literally turn a small aircraft upside down and backwards. Big Jets are much less susceptible, but can still be thrashed around violently in a storm. The pilot has to react to these upsets and keep flying. So, for example, if the winds put you into a stall, you have to recognize that and do what is necessary to keep air moving over the wings properly. Sometimes, the necessary actions might be completely unintuitive or different than what is normal. For example, if you are upside down, some of the controls work in reverse. If you do not know that, or are not practiced in flying upside down, then making that adjustment can be difficult.
A big problem in such circumstances is that in a storm you may not be able to see the horizon, so you have no visual reference for horizontal stability. Moreover, the artificial horizon, the instrument that tells you your attitude, has a lag, so what you are seeing on the instrument was the situation 1 or 2 seconds ago, whereas your current situation may have changed. In a spin or other rapidly changing flight condition the artificial horizon will be wobbling wildly and unpredictably and is less useful than normal in determining what the true attitude of the aircraft is. In a non-visible conditions this can lead to disorientation.
In the case of the AirAsia flight, the storm could have been putting them into a stall and the aircrew's reactions were not vigorous enough to correct it. Unless details are released on their control inputs and other data, we have no way of knowing. Also, they may have just taken the wrong actions. For example, if you are in normal stall you push the stick forwards to get the nose down, but if you are nose up and upside down, then you have to pull the stick backwards to get the nose down. Since you cannot see, it can be difficult to tell if you are upside down or not.
Also, sometimes a pilot will panic if they are losing altitude fast and pull back on the stick, a fatal reaction. Being inside a storm can be very frightening, even to pilots experienced with flying in them (such as meterological pilots), so panic is a real possibility. Once a person becomes panicked, their training goes right out the window and they can do very illogical things. I was once in aircraft with a high-hour CFI who panicked and he did some crazy things.

Thanks Tyler. I am not specifically referring to the AirAsia crash but rather am just interested in what kinds of weather conditions might be so unattractive as to make a pilot take extreme and possibly dangerous evasive action prior to entering those conditions. I'm interested because various 'fear of flying' resources reassure nervous fliers that modern passenger planes are built to withstand any weather conditions they are likely to encounter. This appears to be borne out by what you are saying but instead places the focus for crashes firmly on pilot error.

@Tom Well, first of all, icing can bring down any plane if it is severe enough, so pilot error is not the only factor. You never have to take "extreme" maneuvers to avoid a storm front, you just turn around. A plane can fly a lot faster than a storm. Most crashes involve a pilot making one or more mistakes, however, take into consideration that flying is very complicated, so expecting pilots to make perfect decisions without exception in a crisis is not realistic.

"For example, if you are upside down..." If you are upside down in an airliner, you probably have bigger problems than the stall condition (unless maybe you're Tex Johnson.)

Does an AI really have any non-trivial lag? A gyro-based AI would tumble in an upset, and has minor lag from erection, but should otherwise be accurate in real-time. A glass AI should update pretty quickly, without those flaws.

@NathanG Not only does the "AI" lag, it lags by different amounts depending on what you are doing. Lazy turns can cause really long lags. During upsets or gusty conditions it will rock back and forth and be very difficult to use.

That's news to me. Asked as a question to get more info: http://aviation.stackexchange.com/q/11924/136

QZ8501 was no Cessna 172 with 40 year old mechanical attitude indicator. It was A320 with AI fed by inertial reference system that can do pretty precise dead-reckoning and is used for flight controls and with alpha-limit that already saved many planes from crashing when they approached stall in heavy turbulence. If they really stalled, which we can't know for sure until NTSC publishes FDR data, there must have been a technical problem in addition to whatever mistake pilots may have made.

Realistically I can only think of one: downburst on final approach. In downburst the aircraft in relatively quick succession encounters headwind, downwind and tailwind. The increase of headwind causes increase in airspeed, but loss of kinetic energy due to increased drag. Then the downwind simply causes loss of altitude and finally the change from headwind to tailwind causes loss of airspeed that may be easily large enough to stall the aircraft.
If this dangerous phenomenon is encountered at low altitude (it only occurs at low altitude, too), the pilot must fly the aircraft at the edge of stall to preserve as much height as possible and hope the engines will be able to make up for the lost energy before he runs out of it. It is the reason Airbus introduced alpha-limit on A320 and alpha-protection already earlier. Alpha-limit limits angle of attack to just below stall when the side-stick is moved all the way aft and alpha-protection sets full power when a slightly lower angle of attack threshold is exceeded.
I suppose this is not what you meant. No, I can't think about anything at high altitude that would call for such aggressive manoeuvre. At altitude either you see the weather developing ahead for couple of minutes (with eyes or on weather radar) and avoid it using normal manoeuvres or you hit unexpected clear air turbulence and just react to it. And clear air turbulence is quite likely to make the plane loose altitude, hundreds of feet, rarely even couple thousand feet, but not gain it.

Relevant reading, perhaps, for those interested: https://en.wikipedia.org/wiki/Delta_Air_Lines_Flight_191

"And clear air turbulence is quite likely to make the plane loose altitude, even couple thousand feet, but not gain it." Unless it confuses the alpha protection, which then sends the plane into a steep, uncommanded climb.

"Even couple thousand feet" - excessive, surely? I've never heard of clear air turbulence causing more than a couple of hundred feet of altitude loss. A microburst can cause more of a drop, but that's not clear air tubulence

@JonStory: Usually hundreds, but thousands seem to happen occasionally. In this incident it was 7500 ft. That would include altitude lost while recovering from any attitude the turbulence got them in.

Tailwinds causing a loss of  indicated airspeed is a myth

@rbp: So what happens when the headwind changes to tailwind as you pass through a downburst according to you (I never claimed anything about tailwind in general, only in context of downburst; I'll try to make it more clear that I don't)?

you're confusing updrafts (plane speeds up at same altitude/power) and downdrafts (plane slows down at same altitude/power) with head/tail wind http://en.wikipedia.org/wiki/Microburst#Danger_to_aircraft


text:
Why do airplanes lift up their nose to climb?
Is it right that basically an airplane just needs to accelerate to climb?
Greater velocity of an airplane leads to greater lift - and since its weight remains constant (or even decreases) - a greater lift leads to a gain in altitude without the need to pitch. But why do airplanes climb by "pointing the nose up"? Is it to climb as fast as possible and to need less horizontal space?

Well, hopefully someone really smart is going to come along and give a really brilliant and detailed answer.  But in summary:  Tilting the wings up actually causes the wings to create more lift, which is what increases altitude.  Leveling the wings makes the lift equal to the weight of the craft, so you get no altitude change.

@JayCarr not quite, depending on power, an aircraft can both climb and descend with the nose on the horizon, just as it can fly level (or even descend, like in an approach to landing) with the nose high. You're right that increasing the AoA will increase lift though (to a certain extent), there's a sweet spot for climbing the fastest called Vy.

I assume from the question context he meant "full power".  Besides that, power being equal, positive AoA always creates more lift.  So I don't disagree with you, but I think you've stumbled outside of the intended scope...

Oh, but yeah, you're right.  Too much up angle creates problems because the flow starts to separate from the wing...  See, this is why I need someone smarter to answer this ;)

@jay-carr, a briliant anwer would be "because nose-down you'll hit the ground" :-). Is there a jokes area on this site?

The lift depends upon aoa(angle of attack) of the wing so if u pitch up the aircraft using elevator,the aoa will increase so lift will produce o a short run.

Next time you're in a car, hold your hand out the window and tilt it up and down. The tilt directs the airflow, forcing it down which provides more lift, at the expense of using more fuel.

It definitely depends on the airplane. Both the Cessna 400TT and Grumman Tiger, for example, can climb in a very, very flat attitude that doesn't look as if it should be causing a climb.

Watch a BUFF (B-52) take off.  A BUFF doesn't rotate.  Because of the bicycle and outrigger landing gear, he has to take off level.  Very shortly after he breaks ground, he will drop the nose, and climb with the nose actually below the tail.  It definitely takes a little getting used to.

You can even climb with the nose facing down. :)

A nose-up attitude will not give increased lift, as the angle of attack (which is the angle that the airflow hits the wings) does not necessarily change.

Because up is where the sky is. If you want to fly higher, you point the airplane in that direction. Try the hand outside car window demo that was suggested and report back with any remaining confusion.

This may help answer the question -- https://aviation.stackexchange.com/a/82047/34686

See this https://aviation.stackexchange.com/questions/15243/is-excess-lift-or-excess-power-needed-for-a-climb/82047#82047 (and other answers to the same question) to better understand what happens if we try to climb without lifting the nose, i.e. without changing the pitch attitude of the wing

Because that's where the sky is...

A climbing aircraft needs less aerodynamic lift than in horizontal flight, not more.
Now I have your attention, I hope. The reason is quite simple:
Lift equals weight, and just because the pilot chooses a different flight path angle, the weight of the aircraft does not change. The total of all lifting forces must still balance the weight, but in climb you get a small lifting contribution from the engine(s) because its (their) thrust will point upwards just like the rest of the airframe.

Don't let the many arrows and greek letters confuse you. To be in equilibrium, lift (L, dark blue), drag (D, red), thrust (T, green) and weight (m⋅g, black) must add up such that they can be combined into a closed run of vectors. I've done this with the lighter-colored vectors around the weight. Since the flight path points upwards, so does the thrust which now has a small vertical component. The lift vector can be a little shorter now.
Consider the extreme case of vertical climb: Now all thrust supports the weight, and aerodynamic lift is no longer needed.
There is a second, much more subtle effect: When you climb, air gets thinner and engine performance goes down proportionally. At the same indicated air speed, the aircraft will continually decrease its climb speed, and this deceleration frees up a tiny inertial force, which again adds to lift and counteracts weight.
Conversely, at the beginning of a climb phase the aircraft needs to create momentarily more lift to accelerate itself upward. Only then, when climb speed increases, lift must be bigger than weight to overcome the inertial effect which at this moment works downwards. For the supernerds: If you integrate the lift deficit over time of the aforementioned effect and the extra lift over time for climb acceleration, both cancel exactly.
To answer your question directly: To climb you need to increase excess energy, not speed. This is normally done by increasing engine power output, or by trimming the airplane at a lower speed where drag is lower, so more power remains for climbing. This question contains more details on how to get an aircraft to climb. Note especially @SteveV.'s bucket analogy.
If you use the airplane's kinetic energy as its source of thrust, the same mechanism can be applied to instationary climbs, where speed is traded for altitude, like in gliders.
The nose-up attitude is simply the result of a different flight path. Since the required aerodynamic lift will be almost the same, the angle of attack will also be almost the same and the whole aircraft needs to fly nose-up. This is similar to a car which has the same attitude towards the road, but when you drive uphill, both car and road will be tilted upwards.
This analogy breaks down when you change speed - flying at lower speed needs more angle of attack to still create the same lift, and this nose-up change will be added to your attitude angle.

I guess pitching up has two effects: it changes the angle of attack, and it directs the thrust of the engine at a downward angle.  The second factor definitely produces an upward force on the aircraft.  The first may or may not.  On a plane where the engines are mounted to the plane at a fixed angle, you can't separate the two effects.  If you had a Harrier or something where the direction of thrust could be changed without changing the attitude of the aircraft, it would be entirely possible to climb without raising the nose.

@NateEldredge The B52 is able to climb with a nose down pitch attitude.

Hmm... excuse me for being a layman at these things, but doesn't this answer completely collapse on itself as soon as we're looking at unpowered aircraft? A glider can climb by pitching nose-up, and that certainly has nothing to do with thrust...?!? Or have I misunderstood completely?

@DevSolar: A glider can only climb by slowing down, or by flying in raising air. The climb will soon end if no external energy source can be exploited. This answer looks at a stationary climb, which a glider is completely incapable of without external help.

@NateEldredge: Don't confuse the thrust vector with the jet blast or slip stream. Thrust points forward to counteract drag, and pitching up will make the trust vector pitch up, too.

@DevSolar: The answer is still valid if applied to the pull-up maneuver of a glider. With the pitch change the lift requirement becomes smaller because gravity is now acting at an angle to the lift vector. Again, look at the vertical pull-up: No lift required. And as in the case of a powered airplane, the pitch attitude follows the flight path angle. Just use the glider's kinetic energy as its source of thrust. Where would anything collapse here?

As I said, I am a layman (a.k.a. "noob") at these things, so I am not saying you're wrong, just worrying. But I have this nagging feeling that you are getting cause and effect backwards. For one, even when pulling up at 90 degrees, you still get lift from the airflow over the wings, just perpendicular to earth... and even if you would get no lift, that doesn't mean you don't need it in a climb -- that's why Vy is usually not achieved at 90 degrees, isn't it? Because the lift supports the climb?

@PeterKämpf: From an energy-conserving point of view, your answer is correct. It is impossible to gain potential energy without investing some other form (chemical, kinetic). But I do not believe in the 1. law of TD (another discussion). Our disagreement can be tested: Does an airplane climb by accelerating (ceteris paribus) when thrust is perpendicular to weight? If thats the case - and I think it is - then lift is what matters and not some form of excess energy.

@Chris: How would you accelerate, if not by adding thrust? Ceteris paribus means we keep our trim point, so the added thrust will be converted into altitude, not speed. If you retrim the aircraft for a higher speed while adding thrust, it will not climb, but accelerate - just as commanded. I also think that belief has to do with religion, not physics. If you reject the first law of thermodynamics, we two will never understand each other.

@DevSolar: Lift is required in a climb proportional to the cosine of the flight path angle - the rest is covered by thrust (forgetting drag for a moment). At 90° the wing should be at its zero-lift angle of attack - if it isn't, it will soon be, because any remaining lift would pull it sideways (up in the aircraft's system of reference), decreasing the angle of attack. If this explanation is not sufficient, I propose to start a new question. The comments are unsuitable for technical discussions.

Staying with prop airplanes: When we increase power, don't we accelerate the blades (airfoil!), which then produces more thrust(="lift" but used in a different direction) which would increase the "props climb" (again, different direction!)? Therefore, wouldn't the statement: "More speed gives us more lift" be correct?
For gliders: Pulling up in a glider reduces speed and increases climb; the excess energy comes from the speed, but is only used because of the increased angle of attack. The lift coefficient acting parallel to gravity must now become greater as the lift vector is tilted!!!?

Also created a new question as suggested by Peter here:
http://aviation.stackexchange.com/questions/12742/why-do-planes-not-fly-backwards-up-from-the-planes-pov-when-in-a-vertical-cl

@Maverick283: The big difference to a propeller is that all vertical components must balance. In case of a propeller you are right, the blades will produce more "lift", resulting in more thrust. As for the lift vector becoming larger in a pull-up: Think this to the end, a 90° pull-up. Does lift get infinitesimally big? I encourage you to look closer at the picture with the vectors. And the increased angle of attack is wrong: You get a small, brief increase in lift to accelerate the aircraft upwards, but once you climb on a straight path, lift and angle of attack are smaller.

@PeterKämpf: Yeah, that pull up thing is covered in the other question (thanks for the nice answer btw!). one thing I don't quite get yet though: Assuming an airplanes flies with a given speed, straight and level, and then goes into a climb, maintaining the speed, wouldn't the angle of attack necessarily increase as the lift vector does have to increase in order to make the airplane climb? I totally agree that with the change of the flight path the angle of attack will decrease eventually again, yet the airplane can not just "slide" up the air... I believe ;)

@Maverick283: Yes, for as long as the vertical speed increases, the airplane has to be accelerated upwards, and that is done by a AoA increase. That is the "Conversely, at the beginning of a climb phase the aircraft needs to create momentarily more lift to accelerate itself upward" part of my answer. Once the climb is steady, the AoA drops to less than its previous value because due to the tilted flight path the thrust will contribute a little more to carrying the aicraft's weight.

@PeterKämpf: I understand you well; but what I am looking for is an aerodynamic explanation (not a thermodynamic one). Just to understand you right: You say that it is not possible to climb when thrust is perpendicular to weight, right? I personally do not see that this follows necessarily from the lift equation. And not to seem arrogant, but all empirical science is believing.

@Chris: No, thrust must point forward in some way to do its job, the exact direction is unimportant. And all forces must balance, or there remains a resulting force which will accelerate the aircraft. Climb is quasi-stationary, so all forces balance. This is basically Newton's second law. If you increase lift, the aircraft will accelerate upwards. This acceleration will change the angle of attack and reduce lift, or the pilot forces the aircraft into a looping.

@PeterKämpf this is such a great answer. It has actually helped me reframe the entire way I think about lift and aerodynamics into a more consistent mental model. In particular, while pulling up temporarily increases AoA, a sustained climb is not due to excess AoA compared to level flight, but due to an excess angle of airplane to earth compared to level flight (which in turn causes thrust to increase in angle compared to level flight). Suddenly lots of things make more sense. Thanks!

@Chris, "You say that it is not possible to climb when thrust is perpendicular to weight, right?"  It is possible.  Haven't you ever seen a high performance fighter jet climb vertically?  And if you know this but are challenging Peter Kampf, which of his statements asserts this?

A glider on winch launch quickly disproves this. All the winch force is directed downward. Yet the plane climbs because the angle of attack is increased. You are also neglecting drag that acts in a downward direction.

@DJClayworth Please learn about winch launches before posting stuff like this.

@PeterKämpf Already did. How about you. Did you learn about them?

@DJClayworth You must be trolling, but I bite. How can you possibly claim that all winch force is directed downwards? When in the beginning the glider is on the ground and the winch is several hundred meters away? in HORIZONTAL direction? Again, you must be trolling.

I meant "downwards" in the sense of "not upwards". All the vertical component of the winch force is down and not up. Of course there is also a horizontal component. The answer claims that the climbing of the aircraft is due to the upward component of the engine force, but in the case of a winch launched glider this can't be the case.

@DJClayworth The net energy gain of a glider is still positive when the rope is pointing down at an angle more than the arcsine of D/L in forward direction. Take a regular standard class glider with an L/D of 1:38. That arcsine is 1.5 degrees. A line pointing down at 88° means the glider still gains energy! All the answer claims is that the upward component of thrust helps to reduce lift. Maybe you should read the answer again

Consider the relative airflow. When an airplane is not climbing, the relative airflow is horizontal, and so the angle at which the air meets the wings, ie the angle of attack, is measured from the horizon (Case A in the diagram). However, when an air-plane is climbing, the relative wind is tilted downward by the climbing component of the airplane's velocity. If the airplane did not tilt the nose up, the angle of attack would approach zero as the rate of climb increased, reducing lift and efficiency (Case B), so the air-plane must tilt up the nose to keep the angle of attack in an efficient range (Case C). !

@QuadmasterXLII: You are saying that the reason to pitch is to get the angle of attack at which you get Max L/D which is what you want in a climb.

I'd second that.  The F-8 Crusader was very similar to an A-7 Corsair II, but the F-8 had the main wing attached to a hinged mechanism.  This way, they could raise the wing by about 7 degrees, and the AoA, without needing to raise the nose so much.  Most aircraft were hard to land on a carrier because you had to raise the nose so much on approach.  The F-8 typically used the raised wing on approach (not in combat) so they had better visibility over the nose.  This implies that if your wing could change incidence, you wouldn't need to raise/lower the nose of the plane.

While the answer from @Peter Kämpf is all true and sound, i think it misses a point and does not really answer the OP's primary question.

Is it right that basically an airplane just needs to accelerate to
  climb?

Yes this is basically right.
Higher horizontal speed produces more lift so it'll make the aircraft climb.
https://www.grc.nasa.gov/www/k-12/WindTunnel/Activities/lift_formula.html
But it's not the only way to make an aircraft climb. Increasing pitch (while also giving more thrust) is the other and has been explained by Peter.
Which is more efficient?
An aircraft is designed for optimal efficiency at cruise speed and level flight. So you may want to keep your speed within a narrow range around that optimum.
Raising airspeed will also raise drag (to the square of v)
see https://www.grc.nasa.gov/www/k-12/airplane/drageq.html
Drag is what you absolutely want to minimize because it is energy that is completely lost (transformed into heat).
This is why increasing pitch/AoA, while maintaining airspeed constant is the better way to do.
This way drag stays about the same. Of course you still need to provide more thrust (thus energy), as now part of your thrust is directed downward (and part of your lift backward), but you are converting this energy more directly to altitude, eliminating the loss in drag.
So to answer your question, yes it is possible to climb in a strict horizontal attitude by increasing airspeed, but it is more energy-efficient to climb by increasing pitch. (Thrust being increased both ways)

"Higher horizontal speed produces more lift so it'll make the aircraft climb". Yes but only temporarly: Starting to climb while keeping the nose attitude will increase your fpa and decrease aoa, resulting in lower lift. It's only a balloning efect, not a sustained climb

I think this answer creates a false dichotomy.  Regardless of whether you choose to climb at a high (but constant) angle-of-attack, or to accelerate to a high airspeed and keep a low (but constant) attack, the aircraft will be somewhat-nose-high in the climb (unless designed with extreme incidence) because the flight path is aimed upwards.  A third way to climb would be to keep the same pitch attitude that the aircraft had in level (constant-altitude) flight, but this would constrain the angle-of-attack to stay very low, so not much climb rate would be possible.

You can answer this question empirically.
Trim your plane for straight and level flight and set the power to Vy. Look at the AI or visual horizon, and note the pitch attitude.
Now set the power to its Vy climb setting and configure the aircraft (ball, cowl flaps, mixture, prop, etc) to climb configuration, but don't retrim the elevator. Adjust ailerons to maintain straight flight.
The airplane will pitch up by itself to its Vy climb attitude.

I do think that you must consider the type of aircraft here! If I'm a hotshot new F-22 pilot with an obnoxious thrust to weight ratio trying to intercept some baddies and I need to quickly reach altitude, you can bet I'm gonna put the nose up and go like a rocket.

But really though, it's all about velocity vectors. If you want to go up, then travel up! The engines propel in the direction of the nose. (Unless you are that hotshot F-22 pilot from before). Also consider aircraft have speed limits under certain altitudes, and also consider that ole' Bernoulli is not the only reason airplanes fly, Mr. Newton has something to say about this as well.

Because most of the lift comes from the angle of attack (AoA) of the wings. Higher AoA means more lift (up to a point).
Also most aircraft will pitch up as they increase speed due to design.

Pitch and AoA are related by you could use some more distinction here.

As a general rule of thumb and without a long drawn out technical explanation of how and why here is a simple answer that my 8 year old son could grasp. In straight and level flight, if you reduce power without altering the Aircraft's attitude the Aircraft will descend, conversely if you increase the Aircraft's power it will climb. Now the same Aircraft without altering its power settings if you alter its attitude by lifting the nose it will slow down hence with the same power settings in you lower the nose the Aircraft will increase its speed. So your rule of thumb is "power equals height" and "attitude equals speed". Go and take a flying lesson and try it, and you will see what I mean.

Welcome. Does this add new elements to the already existing answers?

Normally in an aircraft you change your altitude using the power. If you increase power, your altitude increases. If you reduce power the aircraft descends. In both cases the aircraft is normally at a near-level pitch angle.
The reason for this behavior is that the wing is permanently tilted upward by a certain amount, called the "chord angle" or "angle of incidence". The angle is chosen so that in normal conditions, with medium power the aircraft will stay at the same altitude. If the wings were flat, the aircraft would tend to descend constantly.
The main exception to the above is when you are taking off and want to gain altitude rapidly for safety reasons. In this case, the stick or yoke is pulled back and the aircraft tilts upward and climbs rapidly. What causes this is the elevator (or horizontal stabilizer) which is located on the tail of the aircraft:

The elevator allows the pilot to change the pitch of the wings. The more wing surface that is exposed to the air, the greater the upward force. You can demonstrate this yourself by holding your hand outside the window of a fast moving car. If you hold your hand level and then tilt the leading edge up, your hand will be forced upward by the wind, and vice versa. If you tilt the leading edge of your hand down, then your hand will be forced down by the wind. The same thing happens to an aircraft.

No, Tyler, level flight is when you stay at the same flight level. When you climb or descent, it's not level anymore. BTW, the F-104 had quite flat wings. I do remember seeing Starfighters climbing just like any other airplane.

@PeterKämpf I have edited my answer to change the phraseology. Nearly all aircraft have a designed positive angle of incidence. Delta wings and some specialized high-performance aircraft are the exception.

In both cases the aircraft is normally at a near-level pitch angle. nope, not even close. you need a certain aplha to generate lift and in climb your pitch is that alpha PLUS the climb slope (gamma): see  diagram in Peter's answer.

@Frederico I am pilot, I know how an aircraft descends and climbs.

(it's Federico, without the "r" or I won't be notified of replies). and I could reply that I analyze flight data for a living, you don't want to go the "truth by autorithy" road.
as per "controlling altitude with power" it was already treated here: http://aviation.stackexchange.com/questions/2980
You can argue that for large airliners, when a FL change is required, the pitch is hardly moving (but it still is), I might agree, but that's far from being a general rule.

The pilot choses a different flight path. This new flight path is going higher in altitude and by that is changing the potential energy. mass*gravity *9.81*delta Hight. We need to fly slower with a lower drag and use the extra energy to climb or we need to increase the power to the propellor to overcome the change in potential energy. When the altitude is changing we need also to increase the speed because of the lower air density. The lower air density is effecting the lift and the trust the prop can deliver for a giving RPM
We can calculate the trust by looking into the force vectors Lift and Weight. When the plane change course the lift vector and weight vector who were  in opposite direction  are in a climb path working under a small angel, the climb rate. To counter the weight we need to increase the lift from lift r1 to a lift r2.  But the result is also a vector drag r1. This drag vector is added to the drag in level flight. As a conclusion we can say that we need to increase the trust to overcome the added drag and we need to increase the lift to counter the weight.

Here is a more accurate way of looking at size of the lift vector in a climb -- https://aviation.stackexchange.com/questions/40921/does-lift-equal-weight-in-a-climb/56476#56476

The answer is quite simple and has very little to do with lift. The useful power from a thrust $F_t$ on a body with speed $v$ is given by $P=F_tv\cos(\theta)$, where $\theta$ is the angle between the direction of thrust and the direction of movement. This is equal to the difference between the angle of attack and the chord angle.
In other words, for a given thrust from the on the propeller, you get the most energy out of it when the propeller is pointed directly into the relative wind (i.e. for a specific angle of attack). The farther apart these directions are, the more power is being wasted. An extreme example of this is slow flight, where full power may still be inadequate to hold level flight at extreme angles of attack.
In other words, while climbing in a level attitude is possible, it is inefficient in the same way that slow flight is, though to a lesser extent. You get more performance with the same power by allowing the nose to pitch up.
The wings also have a most-efficient angle of attack (best lift-to-drag ratio), which further enforces the idea that certain angles of attack are more efficient than others.

a greater lift leads to a gain in altitude

Here's where you've first gone astray.  Lift is actually less than weight in a sustained linear climb.  The fundamental thing that makes a sustained steady-state climb possible is that the Thrust vector is pointing upward rather than horizontally, which is only true when Thrust is greater than Drag.  We'll return to this point later in this answer.

But why do airplanes climb by "pointing the nose up"?

Regardless of whether we choose to 1) climb at a high (but constant) angle-of-attack and a lower airspeed, or to 2) accelerate to a higher airspeed and climb at a low (but constant) attack, the aircraft will be somewhat nose-high in the climb because the flight path is aimed upwards, and the pitch attitude of the fuselage is the sum of the climb angle of the flight path plus the angle-of-attack of the wing minus the angle-of-incidence (i.e. the "rigging angle" of the wing relative to fuselage).
A third way to climb would be to keep the same pitch attitude that the aircraft had in level (constant-altitude) flight, but this would constrain the angle-of-attack to stay very low-- the higher the climb rate and the steeper the climb path, the lower the angle-of-attack would be forced to go.  This is not the kind of feedback loop that leads to high rate of climb!
To understand why, in the artificial situation where the pitch attitude of the aircraft is constrained to be fixed, the angle of the climb path affects the angle-of-attack of the wing, you have to understand that the airflow or "relative wind" felt by an aircraft in flight is exactly opposite in direction to the aircraft's path of travel through the airmass-- which in this case is the path of the climb.  (For simplicity we're assuming no wind or updraft/downdraft -- those things can change the climb angle achieved relative to the ground without changing the "relative wind" felt by the airplane, but that's not really what this question was about.)  Understanding that the relative wind "felt" by an airplane is always exactly opposite to the airplane's flight path through the airmass, is one of the most important things in understanding how an airplane flies.
Therefore even in an aircraft with an unusually high angle -of-incidence like the B-52, the aircraft will be nose-high in a steep climb.
In theory an aircraft even an aircraft with zero angle-of-incidence could generate lift with the fuselage exactly horizontal.  If the flight path were climbing slightly, then the wing would be flying at a slightly negative angle-of-attack, but a cambered airfoil can still create lift in such a situation.  But the aircraft would generate a much higher ratio of Lift to Drag if the wing were at some higher angle-of-attack.  Even though Lift is less than Weight in a climb, a high ratio of Lift to Drag is still correlated to a steep climb angle.  See this related ASE answer to learn why:  Does lift equal weight in a climb?
The highest L/D ratios are generated at relatively high angles-of-attack.  So this is when we will see the steepest climb angle.  The highest climb rate comes at a somewhat lower angle-of-attack, but the aircraft's nose will still be pitched well above the horizon, because of the simple fact that the  pitch attitude of the fuselage is the sum of the climb angle of the flight path plus the angle-of-attack of the wing minus the angle-of-incidence of the wing relative to the fuselage.

Is it right that basically an airplane just needs to accelerate to climb?

No, for a steady-state linear climb at a constant airspeed, the aircraft also has to create more Thrust than Drag, and it also has to point the Thrust vector upwards.
At this point we need to revisit the paragraph beginning "A third way to climb would be to keep the same pitch attitude that the aircraft had in level (constant-altitude) flight".  There is actually another problem here besides the fact that we'd be forcing the wing to fly at a very low angle-of-attack, where the L/D ratio is poor.  The other problem is that the Thrust vector is staying horizontal, and thus a sustained steady-state climb is possible.
(Naturally, we can zoom-climb or even loop a glider with no thrust at all.  In loop or zoom climb the requirement for a close vector polygon of Lift, Weight, Drag, and Thrust (if present) vanishes, so the constraints are completely different than in a sustained steady-state climb.)
Consider the case of an aircraft like the B-52.  The wing is mounted at a high angle-of-incidence to the fuselage to accommodate the "bicycle" landing gear design by allowing a no-rotation takeoff, and to reduce drag in long-range cruising flight.  Even with the fuselage level relative to the airflow, the wing is at an efficient angle-of-attack, with a high L/D ratio.  If the aircraft is creating more Lift than its Weight, does this mean that it is established in a steady-state climb?  No, it means the flight path will curve or bend upwards, causing the aircraft to pitch upwards, which gives the Thrust vector an upward component.  At this point Lift will actually decrease slightly to a value that is smaller than Weight as the aircraft settles into a steady-state climb with Thrust greater than Drag, the nose pointing above horizon, and the Thrust vector pointing upwards and helping to support part of the aircraft's weight.
Note that as we change the angle-of-attack of the wing and change the ratio of Lift coefficient to Drag coefficient, for shallow to moderate climb or descent angles, the airspeed eventually responds in such a way that Lift actually stays almost constant, while Drag varies greatly.  The reason we choose an optimum angle-of-attack for climbing is really not to maximize Lift, but rather to minimize Drag and thus maximize the ratio of Thrust to Drag.  But regardless of whether we've chosen an angle-of-attack that yields a high L/D ratio or a low L/D ratio, if the Thrust vector is pointing horizontally rather than upward, then we aren't climbing - at least not for more than a brief instant.  (More on this later!)
Again, for more on the relationship between  Thrust, Drag, Lift, and Weight in a climb, see the related ASE answer Does lift equal weight in a climb?
A closing note-- an exotic situation which is not characteristic of normal free flight (meaning that the aircraft is not connected by a towline to another vehicle that is providing the thrusting force) was discussed in this related ASE question and answer. The situation involves a wing sliding up and down on a pole attached to a cart.  In this case, even though the thrust vector can be construed to be horizontal, the wing can indeed climb slowly up the pole while maintaining a constant level pitch attitude, but its angle-of-attack relative to the airflow will be decreased as its climb rate increases, causing a self-limiting effect on the climb rate, as discussed in the present answer.
And now a closing note to the closing note-- earlier, we stated "if the Thrust vector is pointing horizontally rather than upward, then we aren't climbing."  We've also noted that a glider can be looped with no Thrust at all.  A powered plane can also be "zoom climbed" even if Thrust is less than drag, but the airspeed will be decreasing.  Note that during the "zoom climb", the thrust line is still usually pointing upward.
Can we come up with a really contrived case where we "zoom climb" without pitching up at all?  Yes we can-- but the climb will be very brief.  For example, let's say that we're pulling out of a loop.  Let's say that we're "pulling" 4G's-- the Lift vector is four times the aircraft's Weight.  Just before we reach a horizontal pitch attitude, the airspeed will typically be decreasing, meaning that Drag is greater than Thrust.  As we continue to pull up, there will be an instant in time where the pitch attitude is exactly horizontal, but Lift is still much greater than Weight.  At that instant, if we relax the back pressure and move the stick forward as needed to exactly freeze the aircraft's pitch attitude, the flight path will still continue to curve upward for a very brief interval of time, until the upward curve of the flight path decreases the angle-of-attack of the wing to the point where Lift vector is equal to the Weight vector, or more precisely, the point where the Lift vector is equal to the component of the Weight vector that acts perpendicular to the flight path.  At that instant the centripetal acceleration is zero.  The linear acceleration cannot be zero-- as we continue to hold the pitch attitude of the fuselage constant, the airspeed will decrease, and then the flight path will curve downward again until it is exactly horizontal.  When the Thrust vector is exactly horizontal, steady-state flight is only possible in the horizontal direction, not in the upward or downward direction.  From the pilot's point of view, what has happened is that we've reach a level pitch attitude and then we've rather rapidly "unloaded" the wing to near 1-G condition and transitioned to approximately horizontal flight.  The fact that the aircraft did climb very briefly with the fuselage exactly level would probably be impossible to detect without special instrumentation.  But yes, technically, it is possible to achieve a very brief interval of climbing flight with the Thrust vector remaining exactly horizontal, and in fact something close to this happens almost every time we transition from a dive to an horizontal pitch attitude, unless we somehow manage to control the throttle in such a way that the airspeed remains exactly constant during the final portion of the pull-out.
It should be clear by now to the reader that this very brief interval of climbing flight with a fixed horizontal pitch attitude, is not the dynamic that we see during any steady-state climb.

But lift is less than weight in a glide, even at Vbg.  See https://aviation.stackexchange.com/questions/56352/what-produces-thrust-along-the-line-of-flight-in-a-glider/56371#56371 .For the climbing case, click through to the link at the end of the current answer.

No an aircraft doesn’t not necessarily need to Have a vertical component of thrust to climb.  An airplane which maintains a constant angle of attack and a thrust line parallel to the relative wind will begin to climb if it accelerates to a greater airspeed.  Now the additional power added to the system by the increase engine thrust will be converted into potential energy as the airplane climbs.

@CarloFelicione can you tell me the best way to open a chat room to continue a conversation, besides the way  I did here which was kind of clunky?  ( https://chat.stackexchange.com/rooms/106037/a-room-to-talk-about-american-british-and-french-usage-of-angle-of-attack-a )

@CarloFelicione anyway your comment is addressed by my answer and my other two answers that are internally linked to it.

@CarloFelicione sure we can loop a glider with no thrust at all.  Is my answer unclear that I am referring to the steady-state case (linear climb with constant airspeed)?

@CarloFelicione -- well, I took your comments to heart and added content accordingly.  Not sure it really helped the answer but -- see last paragraphs.  I thought it was clear enough that brief transitory cases were not of much interest, but I guess it's always good to be complete.

According to Wikipedia and what I recall from my earliest days in training as a private pilot:
Relation between angle of attack and lift[edit]
A typical lift coefficient curve.
The lift coefficient of a fixed-wing aircraft varies with angle of attack. Increasing angle of attack is associated with increasing lift coefficient up to the maximum lift coefficient, after which lift coefficient decreases.
As the angle of attack increases so does lift. Exceeding the Critical Angle of Attack further illustrates this point.

Welcome. I don't think this answer gives anything new over and above the existing answers...

Good grief, almost none of the answers add anything new...  Best answer yet was the "stick your hand out the window of a car" comparison.  (I realized this fundamental truth at a very tender age)  I think sometimes our members provide greatly overwrought engineering explanations, when by the simplicity of the question one can discern that the asker will not be able to digest such information.  Detailed questions deserve detailed answers, simple questions deserve simple answers.


text:
Is freezing of windows normal in airlines?
Recently I was flying on an Airbus A320. While cruising around 37,000 feet, I noticed half of the glass on the window had frozen. But it was a thin layer of ice.
Was it anything to be concerned about? Should I have brought it to the notice of the airline crew?
Edit: Ice formed on the inside of window with a small hole in it. I guess it was the middle layer of glass.

In my experience a reboot usually resolves the issue, but if there's a persistent problem it's worth checking for a patch using Windows Update. Not sure why being in an airplane would make a difference.

@user777 ROFL My dear question is not about Microsoft windows. It's about window of Airplane!

@vsvankhede http://i.imgur.com/38BlfVp.png

Was it anything to be concerned

No

Should I have brought it to the notice of the airline crew?

Yes, but not because it is a safety issue. Tell them so they can deal with it to give the next passenger a nicer experience.
What is happening is no different than a glass of ice water sitting on your kitchen counter. The warmer side of the glass (the outside) becomes cooler because of the cold on the inside and it transfers that coolness to the surrounding air. That surrounding air cools enough to cause the moisture it contained to condense into liquid that sticks to your glass. If the glass were cold enough, it would freeze that condensation. The window on the aircraft is that cold so the condensation freezes.
There is a mechanism under the window to keep this from happening. It is a jar of desiccant that absorbs all moisture (dries the air) in the window area so that there is no moisture to condense in the first place. Often, there is a tiny bit still.
As the desiccant absorbs moisture from the air, it retains it. So periodically, it gets saturated to the point it can't absorb anymore and the desiccant needs to be changed.
Either the desiccant in your window needed to be changed or moist air is bypassing the desiccant on its way to the inside surface of the window. Normally there is a seal between the outside window and that thin plastic window you can touch and the only way for air to get in or out of that inter-window area is through the desiccant. Your window may have a broken seal allowing moist air to bypass the desiccant.
Either way, it is a system designed only to keep the window from forming ice so the passengers can look out the window, it has no safety or structural consequences.

Actually, I believe that the desiccant is to prevent not ice, but corrosion. That is the reason that airliners (barring composite-construction craft such as the 787) have such low cabin humidity.

this answer assumes that the ice was inside the cabin and reading the question I would not be so ready to make that assumption.

@Federico I think he's assuming that the ice is on the inside of the outer window pane, not the inside of the inner window pane. That seems likely enough, though perhaps not guaranteed.

Do you have a reference for the existence of this dessicant pack? Every airliner I can remember riding in had a small hole in the thin inner window so nothing is stopping moist air from migrating into the window space. Though aircraft air is not particularly moist in-flight.

@Johnny That small hole vents one inter pane area to another. Ultimately, all air going into and out of the window system flows through a little tube to a desiccant jar.

"If the glass were cold enough, it would freeze that condensation" - practical demonstration, put a bottle of vodka in the freezer (salt-water might also work if you're not allowed vodka, the point is to keep it liquid even in the freezer). Wait a couple of hours. Pour a glass, wait a minute. Unless the air in the room is really dry you'll see ice form on the outside of the glass from condensed water, exactly as this answer describes.

It is something normal, airplanes are flying at -40º outside and is very common that ice is generated.
You need to be aware that some parts of the airplane which are critical for having a safe flight are having ice protection systems to avoid generation, but ice in the windows is just avoiding you to have a nice view and enjoy the views.
Not a big deal.

Its even colder than that outside.  Typically in cruise you are around -55C SAT for cruise.

Would love to know about the ice protection systems.


text:
What typical improvements in lift/drag are gained using dimple & zig-zag strips?
A golf ball goes twice as far using a dimpled surface. Many European aircraft (especially gliders) use dimple strips or zig-zag tape on propellers, wings, struts etc. Very few US aircraft use these products.
The dimples/holes/perforations are sometimes mechanically stamped onto a part, but more often a metallic or fiber tape is applied to a thick part of the surface. This is using existing after market technology and has nothing to do with "blow holes".
This question is not how it works it is HOW WELL it works!
What typical improvements in lift/drag are gained and are there any drawbacks to their use?

Dimples on golf ball reduces drag and doubles its range.

Dimple perforations or strips and zig-zag tape decrease drag on some aircraft surfaces.

Related: Has research into the aerodynamic properties of non-smooth surfaces suggested any promising directions for future aircraft design? and for dimples: Is a golf ball surface a good idea for wings or fuselage?

This question relates to existing technology using strips - not hypothetical complete surfaces.

Not suggesting anything, and on the second question, there is an answer which mentions turbulators, and indicates the turbulence pattern needs not be on the whole surface as soon as we know the direction of the airflow.

I don't think strips/dimples are particularly effective for increasing lift . They reduce drag by transforming the laminar flow over a surface to a turbulent flow so that the flow remains energised and follows the shape of the object instead of separating.

This question has nothing to do with blow holes. This question is specific to after market dimples and zig-zag tape. It is not how it works it is HOW WELL it works!

I was told, many years ago, that hail damage was the cheapest damage one could get on their aircraft.  The theory being, insurance pays but no work is done on the dimpled surfaces because there was less drag with the dimples.  True or not, I can't say.


text:
What regulations support European vs USA traffic pattern entry?
While reading another Stack Exchange question I became aware that Europe approves several more "standard" (for training purposes only, but encapsulates European maneuvers)  traffic pattern entries than the US. AC90-66A.pdf dated 1993 once authorized more than the Europeans but is superseded by date (2016) to the information in the "Airplane Flying Handbook (FAA-H-8083-3B) Chapter 7 (2016)".
Of course we follow ATC at controlled airports and are not held to any specific pattern at uncontrolled. Most pilots try to be considerate and consistent when possible, and predictable for safety reasons.
Similar but different StackExchange questions include...
Is a full traffic pattern required when there is no traffic at an untowered airport?
How is a traffic pattern oriented, entered and exited?
The illustration below shows an overlay off approved entries. The US is in red per FAA-H-8083-3B, and Europe in blue. Note the US approves a circle to the outside of the downwind that Europe does not. Europe acknowledges a straight in (don't we all do this?) but the US does not apparently acknowledge a straight (perhaps discouraged for safety reasons) - at least not in the latest 2016 FAA-H-8083-3B?
Does ICAO or another regulating authority offer any recommended International standard that the FAA is ignoring?

I don't agree wih your US 45 degree join on downwind.  The 45 degree entry yes, but not the overhead, circle to join.

If you disagree, take it up with the FAA. It is an approved procedure on page 7-5 of  the latest "Airplane Flying Handbook (FAA-H-8083-3B)"


text:
Are there any aviation vehicles for Internet service?
Facebook announced on Wednesday (2018.06.27) that it is ending its Aquila drone program after four years of development. Aquila was a high-altitude platform station (HAPS) system designed to deliver internet in remote locations.
The solar-powered Aquila was designed to fly between 60,000 and 90,000 feet and to stay aloft for up to 90 days at a time, while providing broadband coverage to a 60-mile-wide area on the ground. The drone flew twice in 2016-2017.
Do any organizations or governments have or plan to provide Internet service via a drone or other aircraft?

source

Do you mean heavier-than-air, or does Google Loon count?

If by "plan" you mean something to be executed and not reaching it's target is considered a failure, then the answer is probably no. None of the existing proposals is that firm.

The linked article mentions one project by Google (Skybender) and that again one more by DARPA (mobile hotspots). Since DARPA has less than global ambitions, this one looks likely to succeed.

Related: Is an “atmospheric geostationary satellite” feasible with current technology?


text:
Why are you not allowed to use power sockets in the seats during takeoff and landings?
Is it because the extra strain on the electrical system reduces engine performance?

The electrical load is fairly trivial compared to the total engine power. E.g. the CFM56 engines used on A320 or B737 will produce on order of 20 MW at take-off power (by comparison with turboshaft engines of similar size) while the generators are only rated for 90 kVA, or a bit under ½%.

@JanHudec: Given that passenger flights are commonly loaded to within one-half percent of the maximum allowable weight for a particular takeoff (any capacity that isn't being used is money that isn't being made), even a difference that small could still potentially be critical.

In case of emergencies.
If something unexpected happens during landing, you don't want people to trip over cables, look for their phone or whatever or start packing anything plugged into the socket. This is the same reason you are asked to stow away bags or any other loose items and so on as well.

you definitely don't want to trip on power cables, especially if the airplane is on fire with the only exit in front of you. regulations say that an aircraft has to be evacuated in under 90 sec. power cords interfere with this process.

Until recently it was considered that passengers should be free of distractions at these times, since that is when a survivable incident would be most likely, so appropriate action by the passengers would minimise loss of life.  However this appears to have been overturned more recently because passengers tend to be more distracted by having to put their devices down for a couple of minutes.


text:
What is the smiley face sensor at the stern of the F-4 Phantom?
The F-4 Phantom has an interesting collection of sensors at the extreme rear of the empennage. What are these sensors for? My daughter calls them them smiley sensors but I suspect that is not the proper technical term. Her guesses as to their real purpose:

Detect other airplanes.
Detect smiles.

Which one is it?

There are happy ghosts trailing the skies! :D

@Zizouz212: Tori Amos has got you covered!

Your daughter is right as explained here:

The 'bug-eyed' face of the parabrake door mounting the two
  rear-hemisphere radar warning antennas.

So they do detect other airplanes and probably the smiles of their pilots too as they are getting close to the phantom.

Presumably, it's the "eyes" that are the radar sensors. Any idea what the "mouth" is?

Radar warning antennas technically detect the emissions of other aircraft, SAM systems, etc, rather than the aircraft themselves.  The "mouth" appears to be some sort of vent, APU exhaust maybe?

@DavidRicherby I read somewhere that there is a fuel drain, which is probably vomited by that mouth.

The 'mouth' is part of the parabrake door and it opens so it can not be a fuel drain nor APU exhaust. My guess is that is equalizes air pressure or vents fumes from inside the fuselage. Note the holes surrounding the parachute chamber in this image.

@dotancohen That "open" picture looks like an entirely more frightening mouth.  Wish I could unsee...

Farhan is correct that these "bug eyes" are part of the radar warning system, but unfortunately is incorrect as to what that system actually does.
Radar warning systems are commonly installed on military aircraft.  Their primary purpose is to detect the upper-level radio frequencies employed by ground- and airborne-based active (i.e. radiation-emitting) radars, to include those used by radar-guided missiles such as the AIM-120 AMRAAM.  Though each antenna is omnidirectional, radar warning systems typically consist of multiple antennas (usually four) each mounted pointing in a separate "quadrant" for maximum coverage and to determine where the signal is coming from.
The F-4 Phantom employed various iterations of radar warning systems over the years, to include the AN/APR-36, AN/APR-37, and ALR-46.  The antennas pictured in the question constitute the aft-facing pair of one of these systems; the remaining front facing antennas are mounted one on each wingtip as seen below and captioned in the following image:

Port wing tip with bug-eye RWR dome and position light.

However, radar warning systems are almost universally passive in nature, meaning they do not emit radiation of their own, and can only sense radar signals being emitted by something else (and unfortunately not planes per se, nor smiles).